1. Field of the Invention
The present invention relates to a quench detection method and apparatus of a superconductive conductor. More particularly, the present invention relates to the quench detection method and apparatus of a superconducting cable including a superconducting shield covering the superconductive conductor.
2. Description of the Related Art
As for a conventional quench detection method of a superconductive conductor, there is a known method called a bridge method which detects using a quench phenomenon by means of measuring a voltage changing with a quench (superconductor to normal conductor transition). There is a quench detection apparatus, as a superconducting-coil quench detection apparatus using this method, which detects the quench by applying a voltage to serially-connected two superconducting coils (Japanese Patent Laid-Open No. 9-84252).
As shown in FIG. 5, this superconducting-coil quench detection apparatus 100 includes an inverter circuit 108, a smoothing capacitor 109, a breaker 110 and a protective resistance 111, and has a first superconducting coil 101a and a second superconducting coil 101b serially connected thereto. Both the superconducting coils 101a and 101b have a voltage intermittently applied thereto to be excited by a chopper circuit 102, and have circulating current passed through them by a cyclic diode 103 when no voltage is applied. The voltages between terminals of the superconducting coils 101a and 101b are measured by a first voltage measurement amplifier 104a and a second voltage measurement amplifier 104b via voltage waveform shapers 112a and 112b respectively so as to obtain variations of measured values of the voltage measurement amplifiers 104a and 104b with a subtracter 105. And an unbalanced voltage is detected as a variation of a voltage measured value which is an output of the subtracter 105 to detect occurrence of a quench of the superconducting coils 101a and 101b with a counter 106 and a signal generator 107.
As for an actual transmission system, however, a transmission voltage is as high as several hundred thousand volts or even higher as the case may be. Therefore, it is very difficult to measure electric power by a method of electrically connecting to the superconductive conductor. It is not desirable, from a viewpoint of preventing an accident, to detect a quench in the transmission system by a method requiring an electrical connection. Furthermore, in the actual transmission system, an amount of power consumption by a consumer fluctuates constantly so that a change in power in association with the quench is hidden therein. Therefore, it is not possible to measure transmission power directly and detect the quench of the superconductive conductor in a superconducting cable from its fluctuation. From these points, it is not adequate, in the actual transmission system, to use the quench detection method of measuring the transmission power by electrically connecting to the superconductive conductor in the superconducting cable.
Because of the above-mentioned problems, there is a conventional quench detection method targeting a transmission apparatus for the voltage of several hundred volts or so while there is none targeting transmission system facilities for a high voltage of several hundred thousand volts or an extra-high voltage of a million volts or so.